SH-AWD - Function

Function

Current SH-AWD configurations are all mated to a transverse engine, either a turbo-charged in-line four-cylinder Honda K-Series engine in the case of the RDX, or naturally aspirated V-6 versions of the Honda J-Series engine. The engines are mated to a front-wheel-drive transaxle. There is no center differential. The transaxle is bolted to torque transfer unit that spins a center drive shaft that powers the rear differential unit. The rear differential is a T-shaped device. Power coming from the engine is transferred via a central hypoid gear that delivers power to each rear axle. Power to the each axle is modulated by identical planetary gear sets and electromagnetic clutch packs which can vary incoming power side to side.

As described earlier, SH-AWD can vary torque side to side through the rear wheels. This ability to drive one of the rear wheels with more power (remember the row boat analogy) is accomplished through overdriving the rear wheels with respect to the front wheels. The Acura RL is designed with a third planetary gear set and clutch pack that are packaged into a so-called "Acceleration Device", which is bolted in front of the rear differential unit. The Acceleration Device allows torque to be passed to the rear wheels at a near one to one ratio, but in cornering situations, the Acceleration Device variably increases the torque passed to the rear up to 5%, depending on the cornering situation.

As of early 2010, SH-AWD has been implemented by Acura in two functionally similar, but mechanically different configurations, with and without Acceleration Device. The more complex SH-AWD version uses the Acceleration Device and can be found in:

2005–2008 Acura RL Sedan
2009–2010 Mid-Model Change Acura RL Sedan 2011–2012 Mid-Model Change Acura RL Sedan

The less complex version of SH-AWD omits the Acceleration Device and can be found in:

2007-TBD Acura RDX CUV
2007-TBD Acura MDX SUV
2009-TBD Acura TL SH-AWD Sedan
2010-TBD Acura ZDX SH-AWD Four-Door Sports Coupe

The Acceleration Device allows the rear wheels to be variably overdriven with respect to the speed of the front wheels up to 5.7%. The Acceleration Device in the Acura RL is positioned in front of the rear differential. During normal steady state driving, the Acceleration Device input shaft (from the engine) and output shaft (to the rear differential) spin at almost equal speeds. In a cornering situation, the output shaft spins up to 5.7% faster than the input shaft, effectively speeding up the rear wheels. The electromagnetic clutch packs, based on sensor inputs, will vary the torque sent to the rear differential from side to side providing constant and dynamic balancing of the car handling characteristics.

The SH-AWD configuration with no acceleration device overdrives the rear wheels at a constant 1.7% with respect to the speed of the front wheels.

The Acceleration Device was first introduced in the flagship Acura RL sedan, and continues in the MMC versions of the Acura RL sedan. This more complex configuration appears to be unique among the SH-AWD equipped Acuras. This more complex RL configuration could be considered an exclusive flagship vehicle feature, and not specified for vehicles with lower price points. On the other hand, the later SH-AWD configuration without the Acceleration Device could just be a simpler and less sophisticated version due to vehicle dynamics and price points, or it could represent a more refined, less complex and inherently more reliable, new standard for the SH-AWD configuration. The rationale for the two distinct configurations has not been publicized by Acura.

SH-AWD in the Acura RDX and Acura MDX use a default power split of 90% to the front and 10% to the rear. Like the unit in the RL, both can power the rear wheels with various amounts engine torque. The power split under straight-line, high-speed acceleration varies from vehicle to vehicle. In addition, in cornering situations, dependent on the specific vehicle balance and dynamics, the rearward power transfer ratios in the RDX and MDX differ greatly from the RL and TL sedans.

For example, in comparing the RL to the MDX, in hard cornering, the most power transferred to the rear wheels for the RL is 70%, whereas the MDX is only 50%. In straight line efficient cruising, different model years of the RL can transfer from 70 to 80% to the front wheels, while the MDX can transfer as much as 90% to the front wheels.

Read more about this topic:  SH-AWD

Famous quotes containing the word function:

    Science has fulfilled her function when she has ascertained and enunciated truth.
    Thomas Henry Huxley (1825–95)

    Advocating the mere tolerance of difference between women is the grossest reformism. It is a total denial of the creative function of difference in our lives. Difference must be not merely tolerated, but seen as a fund of necessary polarities between which our creativity can spark like a dialectic.
    Audre Lorde (1934–1992)

    The art of living is to function in society without doing violence to one’s own needs or to the needs of others. The art of mothering is to teach the art of living to children.
    Elaine Heffner (20th century)